This invention relates to the location, in longitude and latitude, of fixed or moving, pulsed or constant wave (CW) radars by a moving observer. It utilizes either measurements of signal pulse repetition frequency (PRF) ratios or radio frequency (RF) carrier ratios to estimate the emitter's geolocation. It does not estimate the PRF or RF rest frequency of the emitter.
Locating stationary emitters in longitude and latitude, and locating and tracking moving emitters, are important electronic support measures (ESM) system tasks. In particular, it is desirable to upgrade current radar warning receiver (RWR) systems to perform this ESM requirement. In accomplishing this upgrade, it is advantageous to use antennas already installed on the aircraft.
When employing existing antennas, in order to avoid field-of-view restrictions it is necessary that only a single antenna be used to make the measurements from which emitter location is derived. A common method for obtaining range from measurements made utilizing a single antenna is to exploit the change in Doppler on the ESM receiver pulse time-of-arrival (TOA) or RF carrier measurements. The Doppler change measurements are related to the target's location by ##EQU1## where v.sub.r =relative emitter--observer velocity
r.sub.o =observer position vector PA1 r.sub.e =emitter position vector PA1 .function..sub.o =emitter rest frequency PA1 .function.=Doppler shifted frequency measured at observer.
In this relation, f can be either the RF carrier frequency or pulse repetition frequency (PRF) derived from the pulse TOA measurements.
Direct use of Equation 1 for target location and tracking requires the estimation of all unknowns on the right-hand side. That means .function..sub.o must be estimated, as well as the emitter range and relative velocity. Since knowledge of .function..sub.o contributes nothing to the emitter geolocation, having to estimate it is generally not desirable.
Yiu-Tong Chan and Frederick L. Jardine in "Target Localization and Tracking from Doppler-Shift Measurements," IEEE Journal of Ocean Engineering, vol. 15, July 1990, show how to avoid the need to estimated .function..sub.o by making an additional measurement, the rate of change of frequency, ##EQU2## However, the estimation of this rate requires an extended receiver dwell, which is best to avoid if the technique is to be used for emitters that are PRF-coherent but not RF-coherent. If the emitter is not RF-coherent, then the frequency .function. appearing in Equation 1 must be the PRF frequency, .function..sub.pr.function.. Measuring PRF is a more involved process than measuring .function..sub.r.function., the RF carrier frequency.
To measure .function..sub.pr.function. a pulse deinterleaver extracts the fundamental pulse repetition time interval t.sub.p from the TOA measurement differences; t.sub.p is the greatest common divisor of the time intervals between all emitter pulses. That is, these time differences are integer multiples of t.sub.p. Certain fundamental sets of them, characteristic of a given type of radar, are called pulse repetition intervals or PRIs. The fundamental pulse repetition frequency is then obtained from .function..sub.pr.function. =1/t.sub.p.
The PRF frequency measurement made by an ESM intercept receiver typically has an error of 1:10.sup.7, while the RF carrier can be measured to 1:10.sup.9. For instance, if a 10 GHz RF carrier frequency is being measured at a 17 dB signal-to-noise ratio (SNR) by an intercept receiver having an effective bandwidth of 20 MHz, and with the signal having a pulse width of 0.1 msec and a PRI of 500 msec, then the measurement off .function..sub.r.function. to 5 Hz accuracy can be made within a 0.15-second receiver dwell after collecting approximately 300 pulses. But the measurement of PRF for the same signal to the same accuracy requires at least 0.5 seconds--i.e., 1000 pulses are needed when the receiver has a TOA resolution of 2.5 ns.
Thus, obtaining a PRF rate of change, which requires a linear time variation of the above measurement, may mean a very extended receiver dwell, and this extended receiver dwell adversely affects the time required to detect the presence of emitters at other frequencies. This time-to-detect is a critical ESM system performance constraint, and, because of it, ESM system requirements typically do not allow the use of the Chan and Jardine method for non-RF-coherent emitters.
Another problem with the Chan and Jardine approach, and also with other current Doppler-only methods, is that signal angle-of-arrival (AOA) information is not available until the emitter is located. AOA is generally required in ESM systems soon after initial emitter detection in order to improve situational awareness and aid with possible threat avoidance. This defect can be overcome for emitters that are both stationary and PRF stable by using an alternative approach that resembles bearings-only, rather than Doppler-only, emitter location. This alternative approach is based on initially estimating emitter AOA with the technique disclosed in Shaw et al., U.S. Pat. No. 5,241,313, issued on Aug. 31, 1993. The emitter can then be located with the derived AOA measurements using well-known triangulation methods.
But the Shaw technique cannot be used for CW emitters. Tsui et al., U.S. Pat. No. 5,315,307, issued May 24, 1994, describe a method for extracting AOA from RF carrier measurements. Such an approach, however, requires the emitter to be simultaneously in the field of view of two antennas, and therefore is not a desirable method when using existing RWR antennas.
A further drawback to both the Shaw and Tsui techniques is that they require the estimation of the equivalent of the rest frequency .function..sub.o. Also, both require that the relative observer-target velocity be known, making neither suitable for moving emitters. Hence, it is not possible to use such techniques to generate AOA for a moving emitter, and to then obtain the emitter track using well-known, bearings-only location methods, such as that described in V. J. Aidala and S. E. Hamel, "Utilization of Modified Polar Coordinates for Bearing-Only Tracking," IEEE Transactions on Automatic Control, vol. AC-28, March 1983.